The invention pertains to methods and apparatus for liquefying cryofuels.
Cryogenic liquefaction of gases can be accomplished through a variety of methods, some involving mechanical cycles with gaseous refrigerants, others making use of cycles using the thermodynamic properties of magnetic materials.
Gases such as hydrogen and natural gas are commonly liquefied by processes in which the gas to be liquefied, also called the process stream, is used as the working fluid in a mechanical cycle such as the Claude cycle, Linde cycle, or Brayton cycle, involving compression and subsequent expansion of the working fluid. Various implementations of the Claude cycle into liquefiers have achieved a maximum relative efficiency or figure of merit of about 37%. A figure of merit (FOM) is a ratio of the ideal minimum work that must be supplied to liquefy a quantity of a gas to the actual work that is supplied to liquefy the same quantity of the gas. The ideal work differs for each fluid that is liquefied.
Liquefiers can also have a working fluid loop separate from the process stream. In such devices, the working fluid or refrigerant is separate from the process stream fluid. The cooling provided by the refrigerants in a thermodynamic cycle cools and eventually liquefies the process stream fluid. The coupling between the process stream and working fluids is normally accomplished by one or more heat exchangers. A regenerative magnetic refrigerator that operates as a liquefier is an example of this type of liquefier, where the process stream is separate from the refrigerant.
A regenerative magnetic refrigerator uses working materials such as magnetic solids whose magnetic order or magnetic entropy depends on temperature and applied magnetic field. With such a magnetic refrigerant, cooling is accomplished by a mechanical cycle as follows: The magnetic refrigerant is adiabatically placed in a magnetic field. The conservation of total entropy in this adiabatic process requires that the refrigerant increase in temperature to compensate for the increased order in the magnetic moments or decrease in magnetic entropy of the magnetic refrigerant due to the external magnetic field. This temperature change is sometimes called the adiabatic temperature change and it can be used to transfer heat to a thermal sink with a corresponding decrease in refrigerant temperature. The magnetic refrigerant is then removed adiabatically from the magnetic field, producing a corresponding temperature decrease. This temperature decrease can be used to accept heat from a thermal load with a corresponding material temperature increase. (The change in temperature of a magnetic material that occurs as a result of an adiabatic change in externally applied magnetic field is called the magnetocaloric effect.) The magnitude of this temperature change is typically about 2 K per Tesla or a total of about 10-15 K for 5-6 T. To increase the temperature span between the average hot temperature or thermal-sink temperature, and average cold temperature or thermal-load temperature, it is normal to use regenerative steps in the cycle. Thus, the basic regenerative magnetic cycle consists of: adiabatic temperature increase upon magnetization; heat transfer to a thermal sink; regenerative heat transfer to decrease the magnetized magnetic refrigerant average temperature; adiabatic temperature decrease upon demagnetization; heat transfer from the thermal load; and regenerative heat transfer to increase the demagnetized magnetic refrigerant average temperature back to the starting temperature of the cycle. Such regenerative magnetic cycles, where the regenerative function of the cycle is accomplished by the solid working magnetic refrigerant, are called active magnetic regenerative cycles. Refrigerators based on such cycles are called active magnetic regenerative refrigerators. Active magnetic regenerative refrigerators (AMRRs) can be used as liquefiers (active magnetic regenerative liquefiers xe2x80x9cAMRLsxe2x80x9d)) to cool a process stream.
Prior art magnetic refrigeration systems for liquefying gases present the following drawbacks:
1) high magnetic fields are required for efficient refrigeration;
2) superconducting magnets in Helmholtz configurations, used in some embodiments of magnetic refrigerators to produce the necessary magnetic fields, are expensive;
3) immersion cooling of superconducting magnets with liquid helium is difficult and expensive;
4) design and construction of high-performance, multi-material magnetic regenerators required for optimal, reliable, long-lived active magnetic regenerative devices with brittle magnetic refrigerants is difficult;
5) design of housings to contain heat-transfer fluids and to control the flow of these fluids through magnetic regenerators with reliable sealing mechanisms is difficult;
6) there are intrinsic limitations on the FOM of various configurations of multistage refrigerators for the purpose of gradually cooling and liquefying a process stream; and
7) high frequency, high efficiency operation has not been achieved.
In view of the shortcomings of the prior art, the present invention provides, inter alia, active magnetic regenerative refrigerator (AMRR) systems including one or more of the following features:
(a) increased efficiency through the use of multiple-stage active regenerative magnetic refrigerators with an external process stream to achieve liquefaction of a gas, instead of a gas cycle that uses the process stream as the working fluid;
(b) increased efficiency through a parallel or series-parallel configuration of multiple refrigeration stages, that effectively pump heat from a thermal load in a process stream to a hot bath at room temperature or other common thermal sink temperature;
(c) increased overall efficiency by recognition that a mathematical optimization of cold temperatures for each stage can permit the total work performed by a multistage liquefier to be reduced;
(d) increased efficiency through an ortho-para catalysis of the process stream continuously as a process stream is cooled;
(e) efficient coupling of a magnetic refrigerant and a heat-transfer fluid in a porous regenerator matrix composed of selected magnetic materials configured in geometries to reduce entropy generation;
(f) for each stage or selected stages, an admixture of magnetic materials comprising the magnetic refrigerants, wherein the admixture is optimized or otherwise configured for a particular operating temperatures of that stage;
(g) counterflow heat exchangers for each refrigeration stage (or selected stages), allowing additional cooling of a process stream with a portion of a regenerator heat-transfer fluid as the fluid is returned to room temperature in parallel with flow through the regenerators, thus allowing a greater utilization of the heat-transfer fluid for more efficient cooling of the process stream (such xe2x80x9cfluid bypassxe2x80x9d of the regenerator is effective because the thermal mass of a magnetic regenerator can be configured to be lower in a higher magnetic field than in a lower magnetic field, so that there is more flow from hot to cold than from cold to hot in a balanced flow regenerator);
(h) in multiple-stage systems, an arrangement of inexpensive, solenoidal, superconducting magnets configured with alternating magnetic field directions that simultaneously enhances core fields in the solenoids and provides a magnetic flux return path for adjacent solenoids reducing stray magnetic fields;
(i) a hexagonal configuration of superconducting magnets in a six-stage embodiment;
(j) conductively cooled superconducting magnets using a multistage, small capacity cryocooler;
(k) a sealing arrangement and housing configuration such that a small, controlled leakage of heat-transfer fluid occurs as heat-transfer fluid flow is directed through the moving regenerators segments; and
(l) a stage comprising an active magnetic regenerative refrigeration (AMRR) device that includes a linked chain or conveyor belt of magnetic refrigerating regenerators or segments, constructed of magnetic materials with tunable ordering temperatures that withstand repeated thermal stress due to heating and cooling, while remaining intact and functional.
Methods and apparatus according to the invention allow, inter alia, cooling of a gas, such as hydrogen or natural gas, to the point of liquefaction utilizing refrigeration by adiabatic magnetization/demagnetization of selected solid magnetic refrigerants in an active magnetic regenerative cycle.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the drawings.